Multi-faceted scanners usually comprising multi-faceted rotating mirrors are employed in well known techniques for erecting optical scanning between a light source and a pohtocell. Typically, a light illuminates a silvered mirror, for example, at an angle of 45.degree. to direct light toward a facet that is reflected from the facet toward the object being scanned. Normally the object reflects this light back along the same path upon a photocell. The duration of the scan corresponds to the time for a facet to pass the light beam along the object being scanned. It is usually preferred that the object path scanned is independent of which facet is then in the light beam path.
In connection with television equipment, it is known to use mirror prisms for image scanning along one dimension, usually for line scanning. Since the advent of television, cameras operating in accordance with the image storage system, the need for such mirror prisms has become greatly increased. Recently television cameras have been designed for operation within the infrared radiation range, for example, within the range of 2 to 5.5 microns. Television cameras operating within this wave-length require mirrors or similar light deflecting optical means for scanning an image. Usually one means, for instance, a light deflecting mirror, is used for vertical scanning image division. Rotary mirror prisms which are generally prisms composed of several plane mirrors such as glass mirrors are conventionally employed by suitably mounting them on a shaft or other rotary support. These mechanically composed rotary prisms are found to have many disadvantages, both as to their optical characteristics and their mechanical reliability. In particular, they have been found mechanically difficult to mount the several planed mirrors so that they accurately form a polygonal shape of predetermined dimensions. For short optical path lengths, slight misalignment of the facets is found to be of little practical significance. However, when the distance between the scanning mirror and the object being scanned is many feet, slight misalignment of the facets results in the path of scan changing from one facet to the other. Such a result is especially disadvantageous when scanning labels with an encoded stripe arrangement. If there is misalignment of the facets one facet might make a perfect scan of the coded stripes while the next facet would register no scan at all or only scan a few of the stripes.
Moreover, it is difficult to mount the mirrors so that they accurately retain their spatial positions when subjected to the stressed of high speed rotation. The last mentioned mounting problem entails a danger of injury to persons close to the spinning mirror prism which is often unavoidable. Obviously when the mirror prism should disintegrate shrapnel is produced which may cause serious injury to a bystander.
Thus, many methods have been investigated to produce multi-faceted scanners so that the materials from which they are composed would have high modulus to density ratio, low thermal expansion, low Poisson's ratio, good workability and possess the ability to be readily polishable or coatable with a substance which in turn can be polished to produce high quality optical surfaces. Unfortunately, the imposition of these material restrictions result in the requirement of a material which is not readily available. Presently, in view of these material restrictions and limitations, scanners are now being manufactured from glass, stainless steel, beryllium and chromium carbide. The latter two materials are the most widely used since they more nearly meet the requirements of the predicated material limitations. Of these two, beryllium is found to best satisfy the material requirements of the predicated material limitations and consequently is found to perform in a superior fashion when employed. However, the use of beryllium to provide multi-faceted scanners in and of itself results in still other problems among which are exorbitant cost of the material and the extreme difficulty of working the material into the desired configurations. Chromium carbide scanners, although not as expensive as beryllium scanners, possess very high density and therefore require in the overall general construction of the scanner a driver motor and bearings which are much heavier and much more costly to provide.
There is therefore a demonstrated need to provide multi-faceted scanner systems which may be precisely machined, inexpensively, and with great facility than known scanner systems enabling these multi-faceted scanners to be considered for employment in a vast number of applications other than military or development laboratories where the exorbitant costs of currently available scanner systems can only be justified.
It is therefore an object of this invention to provide a novel multi-faceted scanning system devoid of the above noted deficiencies.
It is another object of this invention to provide a novel multi-faceted scanner capable of operation at high rotational speeds.
It is another object of this invention to provide a novel multi-faceted scanner system characterized by precise alignment of the facets.
Another object of this invention is to provide a novel scanning system which achieves precise alignment of the different facets with techniques that are relatively easy to perform.
These and other objects are accomplished, generally speaking, in accordance with the principles of the system of the instant invention, by centrifugally casting a thin film plastic against an aluminum preform which is placed in a precisely machined polygonal master in a conventional replica process. The plastic, which may be for example an epoxy material, is applied to the preform-master assembly with centrifugal force causing it to impact in the cavity between the preform and the master to produce a bubble-free layer.
In prior art replica techniques the production of large optical surfaces has been contemplated so that the application of a thin film plastic to the preform is found to be relatively easy. However, the ease of accessibility unfortunately of these surfaces does not always assure a bubble-free plastic layer. Since the thickness of these layers may be only, for example, approximately 0.005 inches, it can readily be seen that to fill the narrow cavity between the master and the polygon preform without the introduction of air bubbles is either extremely difficult if not impossible.
The master of the system of the instant invention should be shaped such that when it is rotated the centrifugal force exerts pressure on the liquid epoxy and compacts it into a bubble-free layer. The master is rotated only during the filling operation on a vertical axis which provides the most convenient arrangement.